(i) Field of the Invention
The present invention relates to a process for preparing the molded articles of a polyimide. More specifically, it relates to a process for preparing, with a high productivity, the molded articles of a polyimide which are excellent in appearance and physical properties.
(ii) Description of the Related Art
A polyimide resin which is a heat-resistant material having various applications has excellent thermal properties, mechanical properties and electrical properties, and for this reason, the polyimide resin can be utilized as an essential material in various fields. For the polyimide resins, many combinations of monomers to be selected are possible, but if heat resistance is required, an aromatic polyimide is desirable. However, as a technique of molding the aromatic polyimide resin, there have been mainly used a molding method which comprises obtaining the molded articles of a polyimide precursor from the solution of the polyimide precursor while the solution is partially imidated, and then completing the imidation. Another method has also been used which comprises first obtaining a polyimide powder, and then molding the powder at a high temperature under a high pressure. In the former method of molding the articles of the polyimide precursor from the solution of the polyimide precursor, a support is coated with the solution of the polyimide precursor, and the solution is then dried by heating to remove a solvent, while partially imidated. In order to obtain the molded articles having a desired shape, the solution is required to be dried to remove the solvent, while the polyimide precursor solution having a low viscosity is always maintained on the support. When it is intended to obtain the molded articles having a plane shape, various known molding means can be applied, but in the case that the molded articles having shapes other than the plane shape are prepared, the molding means are limited. In fact, as techniques of obtaining the molded articles having the desired shape, there have been suggested the so-called centrifugal molding method using centrifugal force, the so-called immersion method using surface tension and adhesive strength, a spray method and the like. The common drawback of these molding means is that continuous molding is impossible. The centrifugal molding method can scarcely be applied to the molding of the articles having a large width, the tubular articles having a small diameter and the thin articles, and the immersion method and the spray method are scarcely applicable to the molding of the thick articles and the tubular articles having a large diameter. Furthermore, the spray method has additional disadvantages, and for example, the thickness of the articles is not uniform, and foam is present in the molded articles.
For the solution of these conventional technical problems, the present inventors have already suggested a method for preparing the molded articles of a polyimide precursor in Japanese Patent Application Laid-open No. (Hei) 5-177689.